This invention relates to imaging apparatus and methods and, in particular, to apparatus and methods for in vivo imaging of microscopic internal regions of a patient's body.
In some conventional systems for in vivo imaging of internal tissues, a probe is inserted into a channel, normally a naturally-occurring channel, in the body to collect signals representative of the tissue surface to be imaged, and pass the signals to image processing means.
Where the regions to be imaged are microscopically small e.g. of diameter of no more than 1.5 mms, it is difficult to achieve satisfactory resolution.
For example, when using an optical imaging technique wherein a probe directs light from a laser source onto a region of tissue and detects light reflected back from the region onto the probe, the resolution obtainable is limited by the transparency of the tissue and also by very high levels of scattering of light in the tissue. Due to such scattering the mean optical path length is at least 4.4 times that expected on geometric considerations.
When using magnetic resonance imaging techniques the problem arises that water molecules have a diffusion coefficient of around 3.5.times.10.sup.-3 m.sup.2 /s at 37.degree. C. in bulk water and the diffusion coefficient of water molecules in body tissue is likely to be much the same. In imaging systems, the time interval between RF excitation of a region and magnetic resonance data acquisition is of the order of 10 ms. Hence, half of the molecules originally excited in one part of the body tissue will have diffused over a distance of around 8 microns before their signals are acquired. Reducing the time interval between excitation and data acquisition requires wider acquisition bandwidths to be used, thus decreasing the signal to noise ratio, and also makes it necessary for stronger spatially encoding gradients to be used.
The water molecule diffusion coefficient can be reduced by restricting the free diffusion of water, but only by a relatively small factor. Alternatively, localised cooling of tissue may slow down diffusion, but this produces only a minor reduction in diffusion before the water molecules begin to form ice.